Molecular theory of nematic liquid crystals viewed as effect of collective excitation in ferromagnetic systems

We develop a microscopic theory of the nematic phase with consideration of the effect of the collective excitation on properties of nematic liquid crystals. The model is based on the Heisenberg＇s exchange model of the ferromagnetic materials. Since the orientation of the molecular long axis and the angular momentum of the molecule rotating around its long axis have the same direction, operators can be introduced to research the nematic liquid crystals. Using the lattice model and the Holstein-Primakoff transformation, the Hamiltonian of the system can be obtained, which has the same form as that of the ferromagnetic substance. The relation between the order parameter and reduced temperature can be gotten. It is in good agreement with the experimental results in the low temperature region, the accordance is better than that of the molecular field theory and the computer simulation. In high temperature region close to the transition point, by considering the effect of the higher-order terms in the Hamiltonian, theoretical prediction is in better agreement with the experiment. That indicates the many-body effect is important to nematic liquid crystals.

Temperature dependence of ratio between dielectric anisotropy and order parameter in fluorinated nematic liquid crystals

Temperature dependence of ratio between dielectric anisotropy and order parameter of fluorinated nematic liquid crystal is investigated by using a semi-empirical molecular orbital package that can accurately calculate an angle between molecular dipole moment and long axis. We optimize the molecular conformations with three semi-empirical Hamiltonians AM1, PM3 and PM5, and then make a comparison between computational results and experimental measurements. It is shown that the results obtained from AM1 method are in good agreement with the measurements. The present study offers an applicable method to predict the dielectric properties of liquid crystal material.

静电场对向列相液晶的影响

用统计物理的方法讨论了外加静电场对向列相液晶的影响。结果表明，电场诱导的向列相取向序参量、双折射和向列相—各向同性相的相变温度的改变是场强的偶函数，在电场较弱时与场强平方成正比。

用自旋波理论研究向列相液晶

采用自旋波理论研究向列相液晶。由于液晶分子的长轴取向与分子绕长轴转动的角动量方向一致 ,可引入算符研究向列相液晶。通过对两分子作用势进行变换 ,得到系统哈密顿的表示形式与铁磁自旋波理论的哈密顿形式相同 ,进而得到序参数与约化温度的关系 ,在低温区与实验结果符合得较好 ,在接近于相变点的高温区 ,利用哈密顿求得相变点 ,与Maier

染料掺液晶随机激光温控特性的研究

实验中研究了不同盒厚(100μm、188μm、300μm)及摩擦取向(双面平行、双面反向、双面垂直)的液晶盒在温度变化下出射随机激光的特性。实验结果表明,随着温度的升高,随机激光的强度降低直至完全消失,继续升温随机激光重新出现。样品表面摩擦取向也会影响染料掺液晶随机激光的温控特性,同向摩擦的液晶盒随机激光消失点温度值较低。因此可以通过改变样品表面摩擦取向将随机激光阈值设置在特定的温度,可应用于远程温度传感领域。

序电效应和电场对向列相液晶分子有序度的影响

液晶盒内分子的排列很容易受外场的影响,对向列相液晶盒施加足够大的电场,液晶分子排列和序参数都将随空间变化,序参数发生变化会有序电效应产生,要考虑序参数的变化我们选用朗道自由能展开式表示液晶的体自由能密度。本文讨论的是平行型液晶盒,利用差分法和计算机模拟计算得到了不同电场作用下向列相液晶分子指向矢和序参数随空间变化的图形规律,并讨论了序电效应和序电极化能对自由能密度的影响。

利用核磁共振氢谱的哈勒分析获得分子序列取向参数（英文）

本文通过全氢液晶材料的核磁共振氢谱(1H NMR)的哈勒关联研究推测了分子序列取向参数,包括随温度变化的双峰距离及全谱图的半高峰宽.通过哈勒拟合并外推所得到的4'-正戊基-4'-氰基联苯(5CB)和混合液晶E7分子取向参数,与其它方法获得的取值一致.

基于序参数与取向涨落的液晶电光开关

向列相液晶因其独特的光学和电学各向异性,广泛应用于光电显示等领域。光通信技术的持续发展,需要液晶材料具有更快的电光响应。通过对6种负性向列相液晶施加垂直于分子平均取向的电场,结合特殊入射方式,系统地研究了电场对液晶取向涨落和单轴序参数的调控。实验结果表明:电光响应的开关时间均小于350 ns;液晶的介电各向异性越大,场致双折射率变化越大,其最大值与所施加的电场大小和向列相液晶的介电各向异性均呈线性关系。